Self-organizing graphene nanodots

(Nanowerk Spotlight) The ultimate challenge of nanotechnology is to control the structure of matter with atomic precision. Modern day computer chips and storage devices are only possible because photolithography has evolved incredibly over the past many decades, making patterning on the scale of tens of nanometer possible.

The better we are at shaping and structuring material on a small scale, the more powerful technology we can dream of.

Unfortunately, the atomic scale is entirely out of range for conventional patterning.

Self-assembly is an intriguing alternative– why struggle to create intricate, unbelievably small patterns, if the materials can do it themselves? All biological organisms are created this way, so why not technology? Self-assembly can be fast, cheap and produce smaller structures than any lithographic technique.

Researchers at Technical University of Denmark, Århus University, IBM (USA) and Brookhaven National Laboratory (USA), reports today in Nature Communication ("Self-assembly of ordered graphene nanodot arrays") that they have achieved nanoscale self-assembly within a two-dimensional layer.

Dosing of ethylene and borazine near a hot iridium surface, leads for self-organising of a two-dimensional superlattice of graphene dots. The dots are each about 2 nm in diameter and are surrounded by a two-dimensional alloy of boron, nitrogen and carbon atoms, with a periodicity of about 4-10 nm, which can be controlled by the concentration of source gases and the temperature. (Image: DTU)

The team introduced two different gases - ethylene and borazine - simultaneously in a vacuum chamber. When in contact with a heated piece of iridium, the gas molecules reorganized into a single layer - and alloy of boron, nitrogen and carbon. Within this layer, carbon atoms self-assembled into graphene dots of uniform size of approximately 2 nm, just around 15 carbon atoms wide.

Even more interestingly, these carbon dots interact with each other and spontaneously form a periodic array, by a process known as self-organization. The researchers found out that by changing the ratio between the two gases the periodicity of the carbon dot array can be controlled as well.

To image the new material, the team used an advanced scanning tunneling microscope at Århus University. This is basically a metallic tip that scans over the surface of a material while collecting information on the shape and electronic properties of the surface – down to the scale of individual atoms.

Researchers at IBM came up with a model for the self-assembly, which suggests that a lattice of islands of a given size is the lowest energy configuration for the system, and that the interaction with the Iridium substrate is playing an important role as well.

The present work paves the way for an extreme form of materials design, providing a radically new material that is expected to have novel optoelectronic properties and a variety of potential device applications, with the additional flexibility of tuning the superstructures through control of the growth conditions.